Low light vision devices are widely used in a variety of applications, such as night vision goggles (“NVGs”). NVGs allow military, police, or other persons to view objects in nighttime or low light environments.
A typical night vision goggle employs an image intensifier tube (IIT) that produces a visible image in response to light from the environment. To produce the visible image, the image intensifier tube converts visible or non-visible radiation from the environment to visible light at a wavelength readily perceivable by a user.
One prior art NVG 30, shown in FIG. 1, includes an input lens 32 that couples light from an external environment 34 to an IIT 36. The IIT 36 is a commercially available device, such as the G2 or G3 series of IITs available from Edmonds Scientific. As shown in FIG. 2, the IIT 36 includes a photocathode 38 that outputs electrons responsive to light at an input wavelength λIN. The electrons enter a microchannel plate 40 that accelerates and/or multiplies the electrons to produce higher energy electrons at its output. Upon exiting the microchannel plate 40, the higher energy electrons strike a screen 42 coated with a cathodoluminescent layer 44, such as a green phosphor. The cathodoluminescent layer 44 responds to the electrons by emitting visible light in regions where the electrons strike the screen 42. The light from the cathodoluminescent layer 44 thus forms the output of the IIT 36.
Returning to FIG. 1, the visible light from the cathodoluminescent layer 44 travels to eye coupling optics 46 that include an input lens 48, a beam splitter 50, and respective eyepieces 52. The lens 48 couples the visible light to the beam splitter 50 that, in turn, directs portions of the visible light to each of the eyepieces 52. Each of the eyepieces 52 turns and shapes the light for viewing by a respective one of the user's eyes 54.
As is known, common photocathodes are often quite sensitive in the IR or near-IR ranges. This high sensitivity allows the photocathode to produce electrons at very low light levels, thereby enabling the IIT 36 to produce output light in very low light conditions. For example, some NVGs can produce visible images of an environment with light sources as dim or dimmer than starlight.
Often, users must train to properly and effectively operate in low vision environments using NVGs for vision. For example, the lenses 48, IIT 36 and eyepieces 52 may induce significant distortion in the viewed image. Additionally, the screen 42 typically outputs monochrome light with limited resolution and limited contrast. Moreover, NVGs often have a limited depth of field and a narrow field of view, giving the user a perception of “tunnel vision.” The overall optical effects of distortion, monochromaticity, limited contrast, limited depth of field and limited field of view often require users to practice operating with NVGs before attempting critical activities.
In addition to optical effects, users often take time to acclimate to the physical presence of NVGs. For example, the NVG forms a mass that is displaced from the center of mass of the user's head. The added mass induces forces on the user that may affect the user's physical movements and balance. Because the combined optical and physical effects can degrade a user's performance significantly, some form of NVG training is often required before the user engages in difficult or dangerous activities.
One approach to training, described in U.S. Pat. No. 5,420,414, replaces an IIT with a fiber rod that transmits light from an external environment to the user. The fiber rod is intended to limit the user's depth perception while allowing the user to view an external environment through separate eyepieces of a modified NVG. The fiber rod system requires the IIT to be removed and does not provide light at the output wavelength of the cathodoluminescent layer. Additionally, the fiber rod system does not appear to provide a way to provide electronically generated images.
An alternative approach to the fiber rod system is to project an electronically generated IR or near-IR image onto a large screen that substantially encircles the user. The user then views the screen through the NVG. This system has several drawbacks, including limiting the user's movement and orientation to locations where the screen is visible through the NVG.
Moreover, typical large screen systems utilize projected light to produce the screen image. One of the simplest and most effective approaches to projecting light onto a large surrounding screen is to locate the projecting source near the center of curvature of the screen. Unfortunately, for such location, the user may interrupt the projected light as the user moves about the artificial environment. To avoid such interruption, the environment may use more than one source or position the light source in a location that is undesirable from an image generation point-of-view.